I rebuilt a turbo(Garrett) with a .36AR turbine housing(tighter scroll- .48 AR is standard) that the rebuilder had sitting on the shelf in his spares room. The turbo spools about 200 rpm quicker. Another option would be ball bearings, if you can retrofit an older turbo with ball bearings. Boost controller? Just set the wastegate to crack at 12psi- I don't see how an external boost controller would help, but who knows. A really good rebuild would help spool very slightly I would think.
Have you tried the ALDA adjustment?

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